Onboard equipment making it possible to estimate the speed of the air with respect to the carrier of the said device exists in the prior art. The carrier is for example an aircraft.
In particular, Doppler LiDARs can be used to measure the speed and the direction of the wind by backscattering of a laser beam on particles of aerosol type carried by the wind. The frequency shift between the emitted wave and the backscattered wave makes it possible to obtain the longitudinal velocity information. The longitudinal velocity is the wind velocity component along the laser sighting axis. Combining measurements arising from at least three non-coplanar sighting axes then makes it possible to access the three components of the velocity vector.
In the field of aeronautics, a Doppler LiDAR customarily generates a laser beam using a laser source, the said beam being focused a certain distance from the aircraft. Particles present in the atmosphere, that is to say aerosols, backscatter the incident beam. The Doppler frequency corresponding to the deviation between the frequency of the backscattered beam and that of the incident beam is detected by an interferometer so as to characterize the speed of the aircraft with respect to the wind.
The frequency shift corresponding to the Doppler frequency is directly proportional to the longitudinal component of the relative speed of the carrier with respect to the air, the longitudinal component being the component along the laser sighting axis.
It is known that the Doppler frequency fDoppler has the value:
      f    Doppler    =            2      ×              V        long              λ  in which expression:                Vlong represents the projection on the laser sighting axis of the vector V corresponding to the speed of the aircraft with respect to the air;        λ represents the wavelength of the emitted beam.        
Doppler laser anemometry is increasingly being used in systems for estimating the speed of aircraft since it makes it possible to perform a direct measurement, remotely, without any protruding element and independently of the conventional means relying essentially on pressure measurements.
A particular operating regime called the monoparticle regime is of interest here. In this regime, the laser beam is strongly focused in such a way that each particle traversing it produces an individually detectable signal.
The signal generated during the traversal of the Gaussian laser beam by a particle is customarily designated by the word “burst”. It can be viewed as the product of a signal of sinusoidal frequency equal to the Doppler frequency and of a Gaussian window. The width of this Gaussian window is dependent on the width of the laser beam at the point where the particle traverses it and on the transverse velocity of the particle at this same point.
Depending on the point at which the particle crosses the beam, the curvature of the wavefronts of the beam can moreover induce a variation in the frequency of the signal backscattered around the Doppler frequency. The frequency-modulated signal is then designated by the word “chirp”.
The relative speed of the carrier with respect to the air can be represented by a vector V expressed in a three-dimensional reference frame. Conventional use of a laser beam makes it possible to obtain only the projection of this vector on the axis of the beam.
The modulus of the vector representative of the speed of the aircraft with respect to the air mass which carries it is called the “True Air Speed”.
To estimate the true air speed, customarily designated by the acronym TAS, a combination of measurements arising from at least three non-coplanar beams is therefore required.
The simultaneous emission of a plurality of beams exhibits numerous drawbacks in practice. In particular, this involves the implementation of several telescopes for emission/reception of the beam whose integration on aircraft may turn out to be constraining and expensive.
Moreover, the distribution of the laser power according to several measurement axes is less effective in terms of detection sensitivity.
Finally, if the measurements are performed in a zone where the velocity of the particles is disturbed by the carrier, the determination of the modulus of the velocity on the basis of measurements performed at various points may turn out to be complex.